These studies are directed at two aspects of neural control of the circulation: 1. Effects on cerebral blood flow and the blood-brian barrier, 2. Neural effects on vasa vasorum of the aorta. During the past year, studies concerned: 1. Quantitation of effects of sympathetic nerves in protection of the blood-brain barrier during severe hypertension; 2. Comparison of 133Xenon and microspheres in measurement of cerebral blood flow; 3. Effects of sympathetic nerves and baroreceptor stimulation on blood flow through vasa vasorum. Research goals for the coming year are to answer these question: 1. What is the effect of possible neurotransmitters, acetylcoline and vasoactive intestinal peptide, on cerebral blood flow? Do sympathetic nerves alter the time course of autoregulatory responses of cerebral vessels to changes in arterial pressure? Does ligation of vasa vavorum reduce blood flow through vasa vasorum (measured with microspheres), produce medial necrosis, and lead to dissecting aneurysm in dogs?